1. Field of the Invention
The present invention relates to a plastic split optical alignment sleeve for optical connect which directly holds and connects optical fiber chords having incorporated therein optical fibers, and to a method for fabricating such a plastic split optical alignment sleeve.
2. Description of Related Art
Development in optical communication technology has introduced an optical fiber system into almost every home, and variegated communication services are becoming possible. For realizing an optical communication network for subscribers, economical optical connectors are required.
As shown in FIGS. 1 and 2, a conventional apparatus for splicing optical fiber chords 5,5 used for optical communication uses a connector including connecting plugs P,P incorporating respective ferrules 2,2, in each of which an optical fiber is inserted, and a split alignment sleeve A, which has engaged therein the connecting plugs P,P at both ends thereof for aligning the ferrules 2,2, the split alignment sleeve being inserted in a connecting adaptor B. Here, many of the ferrules have an outer diameter of 2.5 mm.
FIG. 3 shows a conventional split alignment sleeve A for optical connectors. The sleeve A, made of a metal, like phosphor bronze, or a ceramic such zirconia, has a slit 1 along its length. The sleeve A has an inner diameter slightly smaller than its outer diameter and holds and aligns the ferrules 2,2 due to its spring property or elasticity (FIG. 4).
The connecting adaptor B, as shown in FIGS. 1 and 2, comprises a set of sleeves 3,3, a set of housings 4,4 for supporting the sleeves 3,3, and a screw (not shown) connecting the housings 4,4 at their flange portions. At respective edges of the sleeves 3,3 project engaging portions 3a, 3a which engage respective engaging grooves 8a,8a of frames 8,8 of the connecting plugs P,P.
On the other hand, each of the set of connecting plugs P,P, as shown in FIGS. 1 and 2, comprises a ferrule 2 having incorporated therein a capillary (not shown) through which an optical fiber chord 5 is inserted and held therein; a coil spring 6 arranged behind the ferrule 2 and pushing the ferrule 2 forward; a cylindrical stop ring 7 having an inner step portion which contacts the inside of a rear end of the coil spring 6; a plug frame 8 which engages with the stop ring 7 to restrict the motion of the ferrule 2; a thumb nut 9 fitted around the plug frame 8; a caulking ring (not shown) which fixes by caulking Kevlar in the optical fiber chord 5 to the rear end of the stop ring 7; and a rubber holder 10 engaged in the caulking ring.
To splice the optical fibers in the optical fiber chords 5,5 with the set of the connecting plugs P,P and connecting adaptor B thus-constituted, it is only necessary to insert the tips of the connecting plugs P,P into the both ends of the connecting adaptor B so that the engaging projections 3a,3a can engage with the engaging grooves 8a,8a of the plug frames 8,8.
When this is done, the connecting plugs P,P are connected to the connecting adaptor B such that projecting outer surfaces of the ferrules 2,2 contact the inner surface of the split alignment sleeve A and the projecting inner surfaces of the sleeves 3,3 contact the projecting outer surfaces of the plug frames 8,8 and that the end faces of the optical fibers inserted in the both connecting plugs P,P are pushed toward each other to connect to each other due to a force exerted by the coil springs 6,6 incorporated in the connecting plugs P,P.
As a result, the optical fibers of the optical fiber chords 5,5 are connected to each other at low optical losses.
Hitherto, in splicing optical fibers, end faces of optical fibers have been designed so that they are under a pushing force of 10 N, while the holding force of the split sleeve A for holding the ferrules 2,2 is set to 3 to 6 N. However, if the grabbing force (i.e., tightening force) of the split sleeve A is too weak, the ferrules 2 tend to come out with ease, and bending or slipping tends to occur when vibration is applied. On the contrary, if the grabbing force is too strong, fluctuation in compression fixation of the ferrules 2,2 decreases to obviate the above-described defects while in contrast thereto, the pushing force of 10 N at the respective connecting plugs P,P is lessened too much, causing adverse effects on the connection stability of the connecting plugs. For these reasons, recently, it has been considered desirable that the split sleeve A have a grabbing force of no less than 10 N and a holding force of 1.5 to 4.5 N for grabbing or holding the ferurles 2,2. As described above, split sleeves are important among various components of an optical connector.
As described in the foregoing, conventional split sleeves have been made from phosphor bronze or ceramics and manufacture thereof requires a number of man-hours in, for example a step of grinding the inner surface of the sleeve to perfect roundness, so that the cost of a split alignment sleeve increases, thus preventing economization on or cost reduction of connectors themselves.
On the other hand, there have been approaches to fabricting split alignment sleeves by plastic molding. For example, Yoshizawa et al. fabricated cylindrical pipes from phenol resin by transfer molding and made a slit in them by post-processing to obtain plastic split optical alignment sleeves (Kenkyu Jitsuyoka Hokoku (Investigation Realization Report), Vol. 32, No. 3, pp. 831-842 (1983)). However, the above-described process is not so advantageous from the viewpoint of a manufacturing process, since transfer molding requires post-treatments after molding, such as removal of flashes and the like, and formation of slits by post-treatment is also undesirable from an economic viewpoint. Some recent examples are known in which plastic split optical alignment sleeves are produced by injection molding, which is economically advantageous, of polyphenylene sulfide (PPS) and liquid crystal polymers, which are thermoplastic resins, without post-treatment.
However, these conventional plastic split optical alignment sleeves have been used for so-called multi-mode fibers having an optical fiber whose pore has a large outer diameter, e.g., about 50 .mu.m. the conventional plastic split sleeves are applicable to such multi-mode optical fibers but are inapplicable to so-called single-mode optical fibers currently used in communication and having a core whose outer diameter is 8 to 10 .mu.m.
More specifically, it has been considered impossible to realize plastic split optical alignment sleeves which meet various requirements, such as dimensional accuracy, smoothness of inner surface, mechanical strength, various reliabilities and so on, that are acceptable for splicing single-mode optical fibers. Connection characteristics of an optical connector can be evaluated in terms of connection loss and return loss on the end face of an optical fiber. Under ordinary physical contact (PC) conditions, a connection loss of no higher than 0.5 dB and a return loss of no lower than 25 dB are required. Under advanced physical contact (Ad-PC) conditions, a return loss of 40 dB or higher is required. Furthermore, from a practical viewpoint, it is required that the connection characteristics should not be decreased under various severe conditions. Such environmental stability has not been sufficiently considered with regard to conventional plastic split optical alignment sleeves.